Imaging systems have many applications, such as military targeting and night vision systems, and commercial digital cameras. A typical imaging system generally includes a lens that is configured to focus light or radiation onto a focal plane array (FPA) via an aperture. A shutter is interposed between the lens and the FPA, and operates to prevent a scene from imaging on the FPA. The shutter is typically open most of the time, and is closed for short periods of time during calibration.
To reduce the background noise, the FPA often needs to be cooled to cryogenic temperatures, typically less than 150 K. To reduce the background signal from the baffling, the baffling must either be cooled or a re-imaging optical design must be used in conjunction with an appropriately placed optical cold stop to block the baffling from the field-of-view of the FPA detectors. Such techniques allow a high signal-to-noise ratio to be maintained. However, cryogenically cooled IR imaging systems are relatively expensive and are associated with a number of difficulties related to maintaining cryogenic conditions.
Microbolometers are another type of IR FPA, which operate near room temperature, thereby eliminating the need for cryogenic cooling, as well as reducing cost and complexity of the system. A thermoelectric cooler can be attached to the back of the FPA, and a temperature controller and sensing scheme is employed to stabilize the temperature of the FPA and its housing at room temperature (about 22° C.). However, such a microbolometer cannot achieve the same high sensitivity as a cryogenically cooled imager. The noise due to the IR background signals from the optics, shutter, and baffling are present, just as in the cryocooled imager. In addition, the uncooled imager has added noise due to the ambient temperature uncooled detectors and their readout circuits that is not present in imagers with cooled FPA detectors.
Moreover, microbolometers may suffer from excessive spatial non-uniformity caused by the conjunction of four factors: (1) microbolometer detectors are DC coupled; (2) the steradiance seen by a microbolometer detector varies with the spatial position of the detector in the array; (3) the imaging device, including its optics, shutter, FPA, and baffling, is not iso-thermal; and (4) the setup calibration is done with an improperly placed shutter.
In more detail, a DC background signal, provided by each pixel of the FPA in the absence of an IR scene, is substantially higher in a non-cryogenic system as compared to a cryogenically cooled system. These DC background signals translate to lower signal-to-noise ratio and compressed dynamic range. The DC background signals can be removed, usually by AC coupling or subtraction methods. However, this does not improve the signal-to-noise ratio. In fact, background subtraction often degrades the signal-to-noise ratio.
To compensate for DC background signals, a calibration process can be performed, where the FPA is shielded from IR scenes, and a DC background signal is generated by each pixel of the FPA. The system DC background signal, given by the average DC background signal of each pixel, is provided on a pixel-by-pixel basis. When an IR scene is imaged, an offset attributed to the known DC background signal is accounted for, and the actual signals attributed to the imaged IR scene are computed, thereby providing a corrected scene image signal.
An imaging system's DC background signal can be periodically determined in the field by providing a shutter for blocking the camera aperture. The calibration procedure is typically performed each time the imaging system is powered up, so that a new system DC background signal is available for each power-up session. It is also typical that an imaging system be recalibrated each time any of the imaging optics of the system are changed or repaired, as well as at periodic intervals during normal imaging operation.
Conventionally, the location of the shutter in an uncooled IR imaging system is at the rear of the lens block. Resulting offset correction cannot, therefore, entirely correct for internal flux. Rather, some internal flux from the camera is blocked during the shuttering event so that temperature differences between the shutter and the internal housing result in non-uniformity (between the pixels of the FPA) after shuttering. In addition, blemishes or other imperfections in the FPA sensing window are more pronounced when the shutter is physically located between the lens and the FPA.
What is needed, therefore, is a shuttering technique that enhances uniformity correction after shuttering.